Spectrophotometric determination of equilibrium constants of two mutual competitive reactions by the method of isosbestic points

Quantitative Kinetic Modeling in Photoresponsive Supramolecular Chemistry: The Case of Water-Soluble Azobenzene/Cyclodextrin Complexes

“…Indeed, in the hypothesis that both complexes 1:1 and 2:1 form, the relative concentrations of AZO (lower than for NMR) and CD (reaching a much larger excess) used for the UV-visible technique would shift the system even further in favor of the 1:1 complex. Furthermore, as we will see in the following section, isosbestic points 22 The Job plot for AZOt in presence of CD is different with a maximum at XγCD = 0.33, pointing to a clear 2:1 stoichiometry ( Figure 2b). The possibility that γCD could complex two guest molecules has already been reported in the literature, linked to its large cavity 23 .…”

“…Indeed, in the hypothesis that both complexes 1:1 and 2:1 are formed, the relative concentrations of AZO (lower than those for NMR) and CD (reaching a much larger excess) used for the UV−visible technique would shift the system even further in favor of the 1:1 complex. Furthermore, as we will see in the following section, isosbestic points 22 observed in the UV−visible spectral evolution also point to a simple binary mixture. The concentration of a 2:1 complex, if any, would be negligible, and the recorded data representative of the 1:1 complexation would allow a reliable determination of the corresponding association constant.…”

Importance of isosbestic point in spectroscopy: a review

“…[70] Estimation of the molar absorption coefficient of hydrated electron. [71] The determination of the Reaction Rate for the first step in a consecutive reaction A → B → C by absorbance measurements using the wavelength of the isosbestic point of B and C. [72] The existence of isosbestic point in the spectrum of parallel or consecutive first order reaction at a certain wavelength lead to time-independent [73]. Evaluation of the site of the isosbestic point and using it to estimation the minimum DNA concentration causing it to shift.…”

Ultraviolet and Light Absorption Spectrometry

“…A method for calculating the stepwise stability constants of complexes has been reported that uses the characteristics of isosbestic points appearing in the spectra of a series of solutions to resolve the overlapping stepwise equilibria into single equilibria which are then processed by an equilibrium-shift method (323). Another paper describes a method using the two isosbestic points of a dual-ligand, single-metal system to determine the equilibrium constants of complexes in which two ligands compete for the same metal and one complex does not absorb at the measurement wavelength (324). Glenn's method of orthoganol functions has been extended to the determination of dissociation constants of some weakly acidic and basic drugs (325).…”